Amplifier modulation

ABSTRACT

In general, the invention is directed to an efficient amplifier for use in radio-frequency identification (RFID) applications. In particular, the invention provides a highly efficient amplifier that requires little power, yet has significant modulation bandwidth to achieve high data communication rates. The amplifier makes use of many components of a class E amplifier including a first transistor, an inductor coupling the first transistor to a power supply, and a shunt capacitor connected in parallel to the first transistor. A second transistor is connected in parallel to the first transistor. A controller selectively controls the first and second transistors to achieve amplitude modulation at a high modulation bandwidth.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. Ser. No. 09/978,329,filed Oct. 15, 2001, now allowed, the disclosure of which is hereinincorporated by reference.

TECHNICAL FIELD

[0002] The invention relates to radio-frequency systems, and moreparticularly, to amplifier modulation in radio-frequency systems.

BACKGROUND

[0003] Radio-Frequency Identification (RFID) technology has becomewidely used in virtually every industry, including transportation,manufacturing, waste management, postal tracking, airline baggagereconciliation, and highway toll management. One common use of RFIDtechnology is in an Electronic Article Surveillance (EAS) system that isto protect against shoplifting or otherwise unauthorized removal of anarticle. In particular, an EAS system may be used to detect the presenceof EAS markers (tags) that pass through an energizing field. Retailoutlets, libraries, video stores and the like make use of RFIDtechnology in conjunction with EAS systems to assist in assetmanagement, organization, and tracking of inventory.

[0004] A typical RFID system includes RFID tags, an RFID reader, and acomputing device. The RFID reader includes a transmitter that mayprovide energy or information to the tags, and a receiver to receiveidentity and other information from the tags. The computing deviceprocesses the information obtained by the RFID reader. In general, theinformation received from the tags is specific to the particularapplication, but often provides identification for an item to which thetag is fixed, which may be a manufactured item, a vehicle, an animal orindividual, or virtually any other tangible article. Additional data mayalso be provided for the article. The tag may be used during amanufacturing process, for example, to indicate a paint color of anautomobile chassis during manufacturing or other useful information.

[0005] The transmitter outputs RF signals that create an energizingfield, from which the tags receive power, allowing the tags to return anRF signal carrying the information. The tags communicate using apre-defined protocol, allowing the RFID reader to receive informationfrom multiple tags in parallel, or essentially simultaneously. Thecomputing device serves as an information management system by receivingthe information from the RFID reader, and performing some action, suchas updating a database or sounding an alarm. In addition, the computingdevice serves as a mechanism for programming data into the tags via thetransmitter.

[0006] To transfer data, the transmitter and the tags modulate a carrierwave according to various modulation techniques, including amplitudemodulation (AM), phase modulation (PM), frequency modulation (FM),frequency shift keying (FSK), pulse position modulation (PPM), pulseduration modulation (PDM) and continuous wave (CW) modulation. Inparticular, the transmitter makes use of an amplifier, typically aClass-A or a Class-A/B amplifier, to drive an antenna with a modulatedoutput signal. These amplifiers may require significant power tocommunicate with the tags. An amplifier may require, for example, 10watts of power to produce an RF signal having a single watt of power. Inother words, a conventional reader may dissipate over 9 watts of powerto produce a single watt of output, resulting in approximately 10%efficiency. The heat dissipation requirements and power consumption ofsuch an amplifier are not well suited for a number of applications,including those that require a low-cost, hand-held RF reader.Consequently, conventional hand-held readers may have smaller poweroutputs, such as 100 milliwatts, but have limited communication rangesand similar power inefficiencies.

SUMMARY

[0007] In general, the invention is directed to an efficient amplifierfor use in radio-frequency identification (RFID) applications. Inparticular, the invention provides a highly efficient amplifier thatrequires little power, yet has significant modulation bandwidth toachieve high data communication rates. The amplifier incorporates manyelements of a Class-E amplifier, yet overcomes bandwidth and otherlimitations typically inherent in such an amplifier.

[0008] In one embodiment, the invention is directed to an apparatus forproducing an amplitude modulated RF signal to communicate with an RFIDtag. The apparatus makes use of a class E amplifier that includes afirst transistor. A second transistor is used to connect a current pathin parallel to the first transistor. The current in this path may belimited by a series resistor or other means. A controller selectivelycontrols the first and second transistors to achieve 100% amplitudemodulation at a high modulation bandwidth.

[0009] In another embodiment, the invention is directed to an apparatusfor producing an amplitude modulated RF signal having less than 100%amplitude modulation, such as 10% amplitude modulation. The apparatuscomprises a class E amplifier having a first transistor and an inductorcoupling the first transistor to a supply voltage via a first resistor.A second transistor is connected in parallel to the first resistor. Acontroller is coupled to the first and second transistors. By activatingand deactivating the second transistor, the controller varies the supplyvoltage and causes amplitude modulation of the produced RF signal.

[0010] In another embodiment, the invention is directed to aradio-frequency identification (RFID) reader that comprises an amplifierthat produces an amplitude modulated signal. The amplifier includes aninductor coupling a first transistor and a shunt capacitor to a powersupply via a first resistor. A second transistor within the amplifier isused to connect a current path in parallel to the first transistor. Athird transistor is connected in parallel to the first resistor. Acontroller selectively controls the first, second and third transistors.The RFID reader includes an antenna to receive the amplitude modulatedsignal and output an RF communication.

[0011] In another embodiment, the invention is directed to a method ofgenerating an amplitude modulated signal. A first transistor of a classE amplifier is modulated at a frequency for a first period of time. Whenmodulating the first transistor, a second transistor connected inparallel to the first transistor is deactivated. The first transistorand the second transistor are then simultaneously deactivated andactivated, respectively, for a second period of time.

[0012] The invention provides many advantages. Unlike conventionalClass-E amplifiers that are limited to relatively narrow modulationbandwidth, the inventive amplifier described herein is able to achievesubstantially increased data transmission rates. In particular, thelarge inductance of a conventional Class-E amplifier resists rapidamplitude modulation of the current passing through it, thus rapidamplitude modulation of the RF energy produced by the conventionalClass-E amplifier is resisted. By utilizing a second transistor inparallel with the first transistor, and selectively activating anddeactivating the transistors, the current of the inventive amplifierdoes not decay and rebuild during modulation, as with conventionalClass-E amplifiers, but rather, the current level remains relativelyconstant. In addition, the inventive amplifier requires less power thanother amplifiers typically used to achieve higher bandwidth.Accordingly, the invention provides reduced heat dissipationrequirements, thereby reducing the need for costly heat sinks andbatteries. Accordingly, the amplifier may be used in a fully portable,hand-held RFID reader that can conform to RFID standards requiring ahigher modulation frequency.

[0013] The details of one or more embodiments of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0014]FIG. 1 is a block diagram illustrating an example Radio-FrequencyIdentification (RFID) system.

[0015]FIG. 2 is a block diagram illustrating an example RFID reader.

[0016]FIG. 3 is a schematic diagram illustrating an example amplifierfor use within the RFID reader.

[0017]FIG. 4 is a graph illustrating an exemplary amplitude modulatedsignal.

[0018]FIG. 5 is a schematic diagram illustrating another exampleamplifier for use within the RFID reader.

[0019]FIG. 6 is a graph illustrating another amplitude-modulated signal.

DETAILED DESCRIPTION

[0020]FIG. 1 is a block diagram illustrating an example Radio-FrequencyIdentification (RFID) system 2. RFID system 2 includes an RFID station(“station”) 4 that interacts with tag 14 via radio frequency signals 16.In particular, station 4 includes RFID reader (“reader”) 12 thatprovides energy to tag 14, and receives information from tag 14, byproducing and receiving RF communications 16 via antenna 15. Reader 12and tag 14 communicate using RF communications 16 that are amplitudemodulated according to a defined protocol, such as the ISO/IEC 14443 andISO/IEC 15693 standards specify.

[0021] Station 4 provides a workstation or other computing environmentfor processing the information received from tag 14, and for providingtag programming information to reader 12. Station 4 includes, forexample, processor 8 that communicates with reader 12 via communicationlink 13. Reader 12 may be internal to station 4, as illustrated, or maybe external or even a hand-held, portable reader. Accordingly, link 13may be an RS-232 serial communication link, a wireless link, or othersuitable connection for exchanging information with reader 12.

[0022] Processor 8 maintains data 11 that represents a compilation ofthe various articles, tags and associated information. An operatorinteracts with station 4 via input device 10 and display 6. Processor 8receives the input from the operator and, in response, updates data 11.The operator may provide, for example, identity or other informationdescribing an article to which tag 14 is affixed. Data 11 may contain,for example, a bar code database storing bar code information for thearticles. Processor 8 communicates the information to reader 12 via link13, which outputs appropriate RF signals 16 to program tag 14. Processor8 may store data 11 on any suitable computer-readable medium, such asrandom access memory (RAM), non-volatile memory, a magnetic medium, andthe like.

[0023] As described in detail below, reader 12 incorporates a highlyefficient amplifier that requires little power, yet achieves significantdata rate. In particular, the amplifier incorporates many elements of aClass-E amplifier, yet overcomes the modulation bandwidth limitationsinherent in such an amplifier. Accordingly, RFID reader 12 can conformto RFID standards requiring a wider modulation bandwidth. The efficiencyand reduced power requirements of the amplifier allow reader 12 toachieve reduced size and weight. Consequently, reader 12 may be readilyincorporated into a desktop workstation or a portable hand-heldcomputing device, such as personal data assistant (PDA) or the like.

[0024]FIG. 2 is a block diagram illustrating an example embodiment forreader 12. Communication interface 22 receives programming informationfrom station 4 via link 13, and forwards the information to controller24. During transmission, controller 24 directs amplifier 28 via controllines 25 to efficiently produce an amplitude-modulated signal 27 inaccordance with a modulation scheme, such as 100% amplitude modulation.Amplifier 28 incorporates many elements of a Class-E amplifier, yetovercomes the modulation bandwidth limitations inherent in such anamplifier.

[0025] Coupler 30 receives signal 27 and provides signal 27 to antenna15 for transmission as an RF communication. Coupler 30 also providesreceiver 32 with a signal 29 representative of an inbound RFcommunication received via antenna 15. Receiver 32 extracts digitalinformation from signal 29, typically by demodulating the signal, andforwards the information to controller 24 for communication to station 4via communication interface 22.

[0026]FIG. 3 is a schematic diagram illustrating an example amplifier 28that efficiently produces a 100% amplitude modulated signal fortransmission by antenna 15 (FIG. 2). Amplifier 28 includes a number ofcomponents that are arranged as a typical Class-E amplifier, outlined inFIG. 3 by dotted lines 55. In particular, inductor 40 acts as a currentsource for amplifier 28, and is coupled to a supply voltage 34.Amplifier transistor 50 connects inductor 40 to ground, and may be ametal oxide semiconductor field-effect (MOSFET) transistor having adrain connected to inductor 40 and a source to ground.

[0027] Controller 24 is connected to a gate of amplifier transistor 50,which is operated as a switch in response to a control signal. Inparticular, application of a positive bias voltage to the gate ofamplifier transistor 50 causes a large current to flow from supplyvoltage 34, through inductor 40 to ground. Shunt capacitor 52 holds thevoltage on the drain of amplifier transistor 50 during on-to-off switchtransition, thereby avoiding switching losses. Resistor 53 representsthe load of amplifier 28, i.e., antenna 15 (FIG. 2). Capacitor 42,inductor 48 and resistor 53 are designed such that the drain voltage ofamplifier transistor 50 falls back to zero prior to the off-to-on switchtransition, again avoiding switching losses.

[0028] In addition to these components, amplifier 28 includes biastransistor 39 coupled to resistor 41, and connected in parallel toamplifier transistor 50 and shunt capacitor 52. As described in detailbelow, bias transistor 39 provides a second path for supply current frominductor 40 when amplifier transistor 50 is open. In particular, a drainof bias transistor 39 is coupled to inductor 40 via resistor 41. Asource of bias transistor 39 is connected directly to ground.

[0029] Controller 24 selectively activates transistors 39 and 50 tocause amplifier 28 to produce an output signal in which an envelope forthe signal is 100% amplitude modulated. In particular, controller 24switches the amplifier transistor 50 at a high-frequency, such as 13.56MHz, while holding open bias transistor 39. Switching amplifiertransistor 50 causes energy to be stored within inductor 40, and currentto periodically flow through amplifier transistor 50. As a result,amplifier 28 produces an output signal having an envelope of 100%amplitude.

[0030] After switching amplifier transistor 50 for a period of time,controller 24 then simultaneously deactivates amplifier transistor 50,and activates bias transistor 39 for a second period of time. Duringthis period, current flows from inductor 40 to ground via resistor 41and bias transistor 39, reducing the envelope of the output signal tosubstantially 0%. In this manner, controller 24 maintains current flowthrough inductor 40 and prevents the energy stored within inductor 40from decaying.

[0031] When controller 24 initiates switching of transistor 50 duringsubsequent modulation cycles, current through inductor 40 is not neededto re-energize inductor 40, since the stored energy in inductor 40 wasmaintained, thus allowing for a shorter rise time from 0% amplitude to100% amplitude. Consequently, amplifier 28 can achieve increased datatransfer rates. In one embodiment, supply voltage 34 provides five (5)volts, while resistors 53 and 41 have resistances of 12 and 24 ohms,respectively. Capacitors 52, 42 have capacitances of 150 and 50picoFarads, respectively, and inductors 40, 48 have inductance of 25 and3 microHenries, respectively. In addition, bias transistor 39 andamplitude transistor 50 may be metal oxide semiconductor field-effecttransistors (MOSFET's).

[0032]FIG. 4 is a graph illustrating an output signal 72 produced byamplifier 28, and which has an envelope that achieves 100% amplitudemodulation. In particular, output signal 72 represents the voltageacross resistor 53 (FIG. 3). The envelope of signal 72 switches betweena maximum voltage (V_(MAX)) and a minimum voltage (V_(MIN)) for a firsttime period T₁. During a second time period T₂, output signal 72 has avoltage of approximately 0 volts.

[0033] The simulation illustrated in FIG. 4 assumes that amplifiertransistor 50 and bias transistor 39, as well as supply voltage 34, areinitially off. At 0.1 μs, controller 24 begins a first cycle ofamplitude modulating output signal 72 by switching amplifier transistor50 at a high frequency, such as 13.56 MHz, e.g., with a 50% duty cycle.In addition, controller 24 maintains bias transistor 39 in an off state.At approximately 4.2 μs, and after switching amplifier transistor 50 fora time period T1, controller 24 simultaneously activates bias transistor39 and deactivates amplifier transistor 50. During this period, currentflows through inductor 40 to ground via resistor 41 and transistor 39,causing output signal 72 to have an amplitude of substantially zerovolts.

[0034] At approximately 6.2 μs, and after delaying for a time period T2,controller 24 begins a second cycle 76 of amplitude modulation. Inparticular, controller 24 deactivates bias transistor 39 and beginsswitching amplifier transistor 50. In this manner, the current flowingthrough inductor 40 is maintained at all times, preventing current decaywhich is typically experienced by conventional Class-E amplifiers.

[0035] As a result, the envelope of output signal 72 more quicklyreaches 100% amplitude during the second cycle 76, and for subsequentcycles, than for the first cycle 74. Consequently, amplifier 28 is ableto achieve higher data rates than conventional Class-E amplifiers. Inaddition, a shorter rise time during amplitude modulation isadvantageous when communicating with multiple tags simultaneously and,in particular, avoiding collisions between communications from thevarious tags. Yet another advantage of biasing the current through theinductor 40 is the attenuation of ringing within output signal 72 thatotherwise is inherent in Class-E amplifiers due to the RLC components.

[0036]FIG. 5 is a schematic diagram illustrating another exampleamplifier 80 for use within reader 12. In particular, amplifier 80produces an output signal having less than 100% amplitude modulation,such as 10% amplitude modulation. Similar to amplifier 28 describedabove, amplifier 80 includes a number of components 62, 64, 66, 68, 70that are arranged as a typical Class-E amplifier, outlined by dottedlines 75.

[0037] In addition to these components, amplifier 80 includes amodulation transistor 60 connected in parallel to resistor 58, whichconnects inductor 62 to supply voltage 54. In particular, modulationtransistor 60 may be a MOSFET transistor having a source and a drainconnected across resistor 58. By activating and deactivating modulationtransistor 60, controller 24 varies the supply voltage received byinductor 62, and causes amplitude modulation of the output signalproduced by amplifier 80.

[0038]FIG. 6 is a graph illustrating a portion of an envelope of outputsignal 82 produced by amplifier 80, in which the envelope achieves 10%amplitude modulation. In particular, the envelope of signal 82 modulatesbetween peak voltage amplitudes of a first maximum voltage (V_(MAX1))and a second maximum voltage (V_(MAX2)). V_(MAX2) may be, for example,approximately 90% of V_(MAX1).

[0039] The simulation illustrated in FIG. 6 assumes that at 0.0 μs (notshown), controller 24 begins a first modulation cycle by switchingamplifier transistor 64 at a high frequency, such as 13.56 MHz, with a50% duty cycle. In addition, controller 24 deactivates modulationtransistor 60, causing current to flow through resistor 58, and supplyvoltage 54 to drop across resistor 58. Consequently, at 3.8 μs theenvelope of signal 82 has a peak voltage of V_(MAX2).

[0040] At approximately 4.8 μs, and after switching amplifier transistor64 for a time period, controller 24 activates modulation transistor 60,causing current to bypass resistor 58, and increasing the envelope ofsignal 82 from V_(MAX2) to V_(MAX1). In this manner, controller 24amplitude modulates output signal 82. In one embodiment, 10% amplitudemodulation is achieved by selecting supply resistor 58 to have aresistance of 2 ohms. The various components of amplifier 80, however,can be selected to achieve other desired modulation schemes.

[0041] Various embodiments of the invention have been described,including embodiments for producing amplitude modulated RF signals tocommunicate with an RFID tag. The apparatus makes use of many of thecomponents of a conventional class E amplifier. Notably, advantages maybe achieved by incorporating the embodiments described above into asingle RFID reader. The reader may, for example, readily supportmultiple modulation schemes, such as 100% and 10% amplitude modulation,by incorporating the embodiments described above. Accordingly, thereader could readily selectively use the embodiments to support avariety of tag types. In addition, the reader may also use the biastransistor while achieving amplitude modulation with the modulationtransistor. This may be advantageous in reducing the rise time during amodulation scheme of less than 100% amplitude modulation. These andother embodiments are within the scope of the following claims.

1. An apparatus comprising: a class E amplifier having a firsttransistor; a second transistor controlling a current path in parallelto the first transistor; and a controller to control the first andsecond transistors.
 2. The apparatus of claim 1, wherein the first andsecond transistors comprise metal oxide semiconductor field-effecttransistors (MOSFET's), and further wherein the second transistor has asource connected to ground and a drain connected to a drain of the firsttransistor by a resistor.
 3. The apparatus of claim 1, wherein theapparatus produces an amplitude modulated signal in response to thecontroller by: for a first period of time, simultaneously switching thefirst transistor at a frequency and deactivating the second transistor;and for a second period of time, simultaneously deactivating the firsttransistor and activating the second transistor.
 4. The apparatus ofclaim 3, wherein the frequency is at least 13.56 megahertz.
 5. Theapparatus of claim 1, wherein the class E amplifier includes an inductorsupplying current to the first transistor, and a shunt capacitorconnected in parallel to the first transistor.
 6. An amplifiercomprising: a first transistor; an inductor coupling the firsttransistor to a supply voltage via a first resistor; a shunt capacitorconnected in parallel to the first transistor; a second transistorconnected to the inductor by a second resistor, wherein the secondtransistor controls a current path in parallel to the first transistorand the capacitor; a third transistor connected in parallel to the firstresistor; and a controller coupled to the first, second and thirdtransistors.
 7. The amplifier of claim 6, wherein the controllerselectively activates the first and second transistors.
 8. The amplifierof claim 6, wherein the controller activates and deactivates the firsttransistor at a frequency, and activates and deactivates the secondtransistor.
 9. The amplifier of claim 8, wherein the frequency is atleast 13.56 megahertz.
 10. The amplifier of claim 6, wherein thecontroller activates and deactivates the first transistor at afrequency, and activates and deactivates the third transistor.
 11. Theamplifier of claim 10, wherein the frequency is at least 13.56megahertz.
 12. An apparatus comprising: a class E amplifier having afirst transistor and an inductor coupling the first transistor to asupply voltage via a first resistor; a second transistor connected inparallel to the first resistor; and a controller coupled to the firstand second transistors.
 13. The apparatus of claim 12, wherein thecontroller activates and deactivates the first transistor at afrequency, and activates and deactivates the second transistor.
 14. Theapparatus of claim 13, wherein the frequency is at least 13.56megahertz.
 15. The apparatus of claim 12, wherein the amplifier furthercomprises: a shunt capacitor connected in parallel to the firsttransistor; and a third transistor controlling a current path inparallel to the first transistor and the capacitor.
 16. The apparatus ofclaim 15, wherein the controller selectively activates the first andthird transistors.